Python is everywhere in education, F# and Elixir are niche languages.

If you want to use F#, you might get lucky with Meadow boards, yet their focus is mostly getting C# to run on their runtime.

Lots of developers already know python, and if you don't it's easy to get rolling.

Ease of use and familiarity to people who aren't principally interested in programming. These are people who want to make things work using microcontrollers.

For pyboard, it's STM32F4 with about 200Kb of RAM. AFAIK, F# and Elixir target VMs (CLR and BEAM) and it seems unlikely that those VMs plus your code will fit on such a chip.